MCTs are claimed by some to be miraculous cure-alls and by others as dietary abominations. The truth is probably a little more complex, as is so often the case with these things. MCT stands for Medium Chain Triglycerides, with the "medium chain" meaning that they are shorter in length than most normal dietary fats (sometimes called Long Chain Triglycerides or LCTs). MCT is a saturated fat, leading many to decry them as unhealthy. However, almost all research into saturated fat focuses on the common Long Chain Triglycerides, not MCT's. It seems likely that the health concerns around normal saturated fat don't apply to MCT's, as they are metabolized in very different ways. MCTs are found in coconut oil, palm kernel oil and butter, as well as being available in pure form. There is reasonable evidence that:

3 MCT and digestive distress

MCT frequently causes digestive problems, including nausea, vomiting, stomach cramps and diarrhea. There may be several factors that influence the level of digestive distress[2]:

Quantity. Higher quantities of MCT are more likely to cause digestive distress. One study found that 50g of MCT caused all subjects digestive problems, but 30g only caused problems in 10% of subjects.

Adaptation. There is adaptation that occurs with consumption of MCT over several days. Two studies showed a reduction in digestive distress over time; in one, subjects were given 34g/day for 7 days and the other, 60g/day for 14 days. By the end the subjects' distress had reduced or disappeared.

Other Foods. Consuming MCT with other foods reduces the digestive distress.

Personally, I found that even after weeks of taking MCT, I could not consume it on its own without incurring dramatic digestive problems. Mixing MCT with other foods greatly reduced, but did not completely remove, the issues.

4 MCT Shake

The recommendation is to take MCT in a Ketogenic Shake with the MCT mixed with milk (cow, soy, or goat), oils (such as canola), plus sugar (or polycose), and protein powder[3]. For the MCT Diet, a shake containing the daily dose of MCT is taking over 6 meals[3]. The dose of MCT start off at 1/3rd the full dose, building up slowly based on digestive tolerance[3]. An example is 108g MCT, 375g skim milk, plus 13g Canola oil.

5 MCT and Athletic Performance

The available evidence suggests that MCT most likely hinders athletic performance, probably because of digestive discomfort.

5.1 Confounding Factors

The effect of MCT on exercise is not easy to study as there are many factors that might influence the outcome:

When is the MCT administered? MCT during exercise is likely to have a different result to MCT taken as part of a routine diet.

Are the subjects on a ketogenic diet? It seems reasonable that Ketoadaptation might change the effect of MCT.

How long is the trial? There are clear indications that adaptations occur over a period of some days that reduce the side effects of MCT consumption.

Are the subjects trained athletes or sedentary? Often trained subjects respond differently to their sedentary counterparts.

What intensity and duration of exercise is used? High intensity exercise usually burns more carbohydrate than fat.

How much MCT is consumed?

Is the MCT taken alone or with other fuels? Typically MCT in isolation causes far more digestive problems than when taken with other foods. Also, taking carbohydrate with MCT has been shown to reduce the levels of Ketones produced.

What is the MCT compared to? Is the MCT compared with the same calorie intake of another fuel, or to no calories?

5.2 Available Research

The table below summarizes the studies on the impact of MCT on exercise.

During exercise, 54% of the consumed CHO and 64% of the MCT were metabolized, contributed 7% (CHO) and 8.5% (MCT) of the energy expended. Both CHO and MCT maintained blood glucose levels better than the placebo.

During the second hour, 72% of the MCT was metabolized in CM-L compared with 33% in M.

6 MCT and Alzheimer's

There is evidence that MCT can help reduce peoples' memory impairment without carbohydrate restriction[17]. A study using elderly dogs showed a long lasting improvement in cognitive ability with MCT supplementation[18]. There are other anecdotal reports of MCT helping with Alzheimer's symptoms[19].

7 Types of MCT

The 'medium chain' in 'medium chain triglycerides' refers to the length of the chain carbon atoms. Generally, chain lengths of 6 to 12 carbon atoms (C6 to C12) are considered 'medium'. The most common source of MCTs is in Coconut Oil, and typically pure MCT is refined from Coconut Oil. The mixture of oils in most MCT reflects that of Coconut Oil. MCT oils are fully saturated.

8 Calories in MCT

An animal study has shown that overfeeding that includes large quantities of MCT produced 20% less weight gain than the same level of overfeeding with LCT[22]. The rats were fed a diet of 50% of calories as fat, with the MCT diet providing 45% of calories from MCT. The rats had similar levels of activity and insulin. This may because MCTs are raise body temperature (thermogenesis)[23]. MCT has is often considered to have a slightly lower calorie density of 8.3 Calories/gram compared with 9.1 Calories/gram for LCT[24]. However, if thermogenesis is taken into account, a value of 6.72 Calories/gram would be more appropriate[24]. This may be critical when using the MCT Diet if calorie intake is restricted. Given the density of MCT is 0.93 - 0.96 grams/ml[25], 15ml of MCT would be 95 Calories assuming 6.72 Calories/gram rather than 118 Calories assuming 8.3 Calories/gram.

9 MCT and Ketone Levels

A 1969 study looked at how ketone levels changed with MCT[26]. Subjects were non-obese healthy adults, mostly on a high carbohydrate diet who did not perform any exercise beyond slow walking after taking the MCT. Unless otherwise stated, the subjects consumed only water after taking the MCT. The study measured breath acetone, which is likely to peak sometime after blood AcAc or BOHB levels. (Note that from another study[27], Acetone at 1.0 µg/100ml breath might indicate blood ketones of BOHB 1.5 mmol/L and AcAc of 0.7 mmol/L, 5.0 µg/100ml might be BOHB 2.7 mmol/L and AcAc of 1.4 mmol/L. See below for details.)

This is the time course of a single 30ml dose of MCT. You can see the peak occurs after 6 hours, though the blood AcAc levels probably peak sometime earlier.

The effects of different doses of MCT on breath acetone. The 10, 30, 50, and 75ml doses produced a linear, proportional increase in Ketones, but the 100ml is disproportionately large. Also note that the peak occurs later for larger doses.

Even though MCT produces ketones without carbohydrate restriction, it is clear from this graph that carbohydrate taken at the same time as the MCT will inhibit ketone production. Note that from a low carbohydrate perspective, this is quite a lot of sucrose that is ingested.

This shows the linear relationship between maximum breath acetone and the sucrose dose.

The effect of 78g of sucrose on the dose/response of MCT on Ketones. The sucrose reduces ketone levels for all doses of MCT, but the effect is greater at higher MCT levels.

The effect of fasting on response to 30ml MCT. The dotted line and triangles show the ketone levels from fasting alone. The ketone levels in response to MCT are larger after fasting, and the study noted that a low carbohydrate response resulted in a larger ketone response than a mixed diet. (There was only one subject on a low carbohydrate diet, so no statistical inference can be drawn.)

The ketone levels after 1mg/Kg body weight. The units are in mg/dl, so divide by 10 to get the more usual mmol (both BOHB and AcAc have similar conversion ratios).

9.1 Calculating Blood Ketone Levels from Breath Acetone

To understand the above studies, we need to be able to estimate Blood Ketone Levels from Breath Acetone. In a study that looked at the correlation between breath acetone and blood ketone levels the results indicated the following relationships between breath acetone in nmol/L and blood ketone levels in mmol/L[27].

10 MCT and Weight Loss

MCTs are sometimes recommended for weight loss. One study compared MCTs with Olive Oil and found a greater weight loss with MCTs[29]. There is some evidence from an animal study that MCTs may be less palatable than other fats[30]. However, an analysis of animal studies indicates that generally, replacing other fats with MCT does not reduce calorie intake[31]. Based on my personal experiences with MCT, I would suspect that any weight loss benefit comes from the appetite reduction that comes from the associated Nausea.

11 MCT Safety

MCT is considered safe at 1mg/Kg of body weight, and MCT has been shown as effectively non-toxic in high dose (9-12g/Kg) animal studies[32].

12 My Experiences with MCT

Below are my thoughts on MCT while on the Ketogenic Diet, with an update further down the page on using MCT as part of a normal diet.

I've found that taking MCT on its own will often produce digestive problems, including nausea and apocalyptic diarrhea. Mixing MCT with other foods can reduce the problems, and I take MCT with cream or other foods.

I found that MCT in larger doses (3+ tablespoons) can produce obvious thermogenesis, resulting in the perception that a cold room is actually overly warm. There is also a sensation of warmth radiating from my stomach, rather like I'd just consumed strong alcohol.

There seem to be changes in mental functioning, especially with larger doses. MCT can alleviate some feelings of tiredness and improve mental acuity, but it can also result in an unpleasant feeling that is hard to describe. The sensation is a little like how I feel when a migraine is imminent.

I've found that MCT taken during exercise seems to impair my running. I performed a simplistic experiment with MCT during a run to try to evaluate the effect.

At the time of the experiment I was adapted to the Ketogenic Diet, having blood ketones above 1.0 for the previous 10 days.

I started the run fasted, with my last dose of MCT at 2 PM the previous day.

For the first 8 miles, I felt really good, with my heart rate and breathing where I'd expect them to be.

At 8 miles I took 3 tbsp. of MCT with 2 tbsp. of Nutella. The Nutella was to prevent GI problems, as I've found that 2 tbsp. of MCT taken on its own can produce debilitating diarrhea. The Nutella contained 21g of sugar, but there is research that indicates that 50g of carbohydrate taken before or after a 20 Km/12.4 mile run at a fast (7:15 min/mile) will not impact ketosis[33].

By mile 10 my breathing had become strained, and I had to slow up slightly to keep things under control. My heart rate was slightly higher, but not enough to explain the breathlessness. My breathing problem peaked at about mile 11, before gradually easing off, though it never returned to normal.

At mile 19 I stopped for a drink and my blood pressure dropped dramatically. Even crouched down with my head between my knees I was dizzy. After a few minutes things returned to some semblance of normality and I was eventually able to stand up and continued to run.

A few days later I repeated the test, but took 3 tbsp. of Macadamia Nut Oil with 2 tbsp. of Nutella at mile 8. My breathing remained normal and I had no other problems.

The comments above are from using MCT during my Ketogenic Experiment in 2013. In late 2015 I started to experiment again with MCT while losing weight on a relatively normal diet. During this time I was eating a high-fiber diet that was predominantly plant based. Over a period of a couple of months I gradually introduced MCT into my diet. Initially I had some minor nausea and digestive disturbances, but they settled down and by the end of the period I was able to consume several tablespoons of MCT without any issues. I would either mix the MCT with a high protein/high-fiber smoothie, or blend the MCT into a little milk and add a couple of shots of espresso. Adding 2 tablespoons of MCT to my coffee before running was possible without G.I. problems. I did notice that while running on a cold morning after taking the MCT I could feel the thermogenic effect. There was a sense of warmth spreading out from my midriff, rather like having drunk a short of strong alcohol. Unlike the mental effects I had when I was on the ketogenic diet, I had no issues this time around.